add PieceTable::Cursor::operator*(), add unit test

src/core/PieceTable.h

@@ -63,6 +63,12 @@ public:
 
         /** Virtual column (0 based). */
         uint32_t column;
+
+        uint32_t operator*() const
+        {
+            /* TODO: Use Encoding */
+            return piece->start[offset];
+        }
     };
 
     PieceTable(const uint8_t * file_buffer, unsigned long file_buffer_size);